Properties of GEMs and the GEM-MGC combination [gas electron multipliers]

Gas electron multipliers, can be used as charge (pre) amplifiers as well as light producing imaging devices. We have examined GEMs as charge amplifiers in several gas mixtures, including Ar-DME, Xe-DME, Xe-CO₂ as well as in pure argon and xenon. The operation of a single GEM, cascaded GEMs and a combination of a GEM and a micro gap chamber (MGC) have been studied using ~6 keV X-rays. The collected charge cloud in the gas mixtures is limited to ~10⁷ electrons using two GEMs in tandem mode with a pad read out, and 2.7·10⁷ electrons using a MGC as read out. These values correspond with a gain of ~10⁵ for 5.9 keV X-rays from ⁵⁵Fe. The stability of the gain in gas mixtures is better than 10% except for Ar/DME 60/40, where the stability depends on the gain. In pure argon and xenon the maximal gain is respectively 4000 and 7000, using two GEMs in tandem mode. The field strength distribution for optimal operation of a GEM is studied in detail